Eosinophils, basophils and mast cells have been implicated as the major cell types producing inflammatory mediators in response to helminthic infections, as well as several diseases, particularly asthma, rhinitis, and atopic dermatitis (Weller, P. F. (1991) N. Engl. J. Med. 324:1110; Sur, S., C. et al. (1993) In Allergy Principles and Practice. E. Middleton et al. eds. Mosby, St. Louis, Mo., p. 169; Costa, J. J. et al. (1997) JAMA 278:1815). In these situations, the preferential accumulation and activation of these cells has been noted. Although considerable progress has been made in our understanding of eosinophil recruitment to the site of inflammation, a number of key points are still unclear, including the exact mediators utilized for localization to these sites during the migration process. For example, activation of microvascular endothelial cells and expression of adhesion molecules, notably VCAM-1, is felt to be a key event in this process during allergic inflammation (Bochner, B. S. (1998) In Allergy Principles and Practice. J. Middleton et al. eds. Mosby, St. Louis). In addition, a number of chemokines and other chemotactic factors, such as those acting via CCR3, have been implicated because of their involvement in eosinophil, basophil and mast cell chemotaxis (Dahinden, C. A. et al. (1994) J. Exp. Med. 179:751; Daffern, P. J. et al. (1995) J. Exp. Med. 181:2119; Nickel, R., L. et al. (1999) J. Allergy Clin. Immunol. 104:723; Romagnani, P. et al. (1999) Am. J. Pathol. 155:1195; Rot, A. et al. (1992) J. Exp. Med. 176:1489). Another possibility, however, is that these cells are selectively recruited and activated in a specific way due to a unique cell surface phenotype. While eosinophils, basophils and mast cells are readily identifiable based on their tinctorial properties, as yet there has been no cell surface marker identified that is unique to these cell subsets (Saito, H. et al. (1986) Blood 67:50; Bodger, M. P. et al. (1987) Blood 69:1414).
Sialoadhesin factor-2, or SAF-2 (European Patent Publication No. EP 0 924 297 A1), is a member of the sialoadhesin family of proteins also known as the I-type lectins and recently renamed the siglec family (sialic acid-binding Ig-like lectins) (Kelm, S. et al. (1996) Glycoconjugate Journal 13:913). The family members include sialoadhesin (siglec-1), CD22 (siglec-2), CD33 (siglec-3), myelin associated glycoprotein (MAG or siglec-4), siglec-5 (Cornish, A. L. et al. (1998) Blood 92:2123), OB-BP-1/siglec-6 (Patel, N. et al. (1999) J. Biol. Chem. 274:22729) and AIRM1 or siglec-7 (Falco, M. et al. (1999)J. Exp. Med. 190:793; Nicoll, G. et al. (1999) J. Biol. Chem. 274:34089). With the exception of siglec-4, all are expressed on various subsets of hematopoietic cells. Siglecs belong to the immunoglobulin (Ig) supergene family and have an N-terminal V-set Ig domain followed by 1-16 C2 set Ig domains. Siglecs mediate sialic acid-dependent adhesion with other cells generally preferring either α2,3 linkages (siglec-1, -3, and -4) or α2,6 linkages (siglec-2) (Kelm et al. supra). Most family members have either immunoreceptor tyrosine-based inhibition motifs (ITIM) or activation motifs (ITAM) that participate in signaling through Src homology 2 (SH2) domain binding to the phosphotyrosine of the ITIM or ITAM. This has been demonstrated for CD22, CD33 and AIRM1 (Falco et al., supra; Freeman, S. D. et al. (1995) Blood 85:2005; Blasioli, J. et al. (1999) J. Biol. Chem. 274:2302).
SAF-2, now known as Siglec-8, exists in two isoforms with identical extracellular and transmembrane sequences. One isoform has a short cytoplasmic tail with no known signaling sequences (Siglec-8), while the other, Siglec-8 long form (Siglec-8L), has a longer cytoplasmic tail containing two tyrosine-based signaling motifs (Foussias, G. et al. (2000) Biochem Biophys Res Commun 278:775; Munday, J. et al. (2001) Biochem. J. 355:489). Although the function of Siglec-8 and Siglec-8L, and indeed most Siglecs, is unknown, the cytoplasmic region of Siglec-8L contains one consensus immunoreceptor tyrosine-based inhibitory motif (ITIM) and a signaling lymphocyte activation molecule (SLAM)-like motif, suggesting that Siglec-8L may possess signal transduction activity.